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At 20° left/right you will be approx -2dB down from max gain. If you also are 20° off axis vertically, then tally another -1dB (approx) loss.

The antenna's vertical and horizontal beamwidth spec reflects the -3dB (half power) crossing points (the blue colored circle in the linked beam plots). Most folks are under the impression that the signal stops there. But, these are not brick walls and useful signals continue for many more degrees. Even the antenna's backside has active fish fin regions that are only -10dB down from max gain. But the deep null areas are indeed dead zones in practice.

The angle is written on the outside of the diagram, so for reduction at 20° right you'll take the horizontal graph, look where the curve is at 20° right (340°), and you'll see it's roughly at -1dB (the dB circles are a log scale). Knowing range is doubled every 6dB you can calculate the reduction easily. Horizontal and vertical effects are summed.

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I understood the 3db plot but found it hard to understand what it realy ment in practice.

The reason I asked is because I am trying to make plans for a new video receiving system that I have in mind. If all goes to plan it will be a significant improvment on my current 4 way diversity system. I won't say too much yet but it draws experience from both my 4 way diversity system and my auto tracking aerial.....